Automobile wheels made of steel, for example, typically have a so-called two-piece structure in which a wheel rim and a wheel disc are welded to each other. It is known that the peripheral edge of a flange portion of the wheel disc and the inner circumferential surface of the wheel rim can be joined with each other by fillet welding. Since the automobile wheels are important safety components, it is desired that the automobile wheels fully exhibit their dynamic properties such as specified strength and durability. It is possible to determine whether or not the wheel rim and the wheel disc are joined with each other with an adequate joining strength by investigating the welding depth into the wheel rim and the wheel disc at a welded portion between the wheel rim and the wheel disc.
In order to examine the welding depth at the welded portion in a manufacturing site of the automobile wheels described above, a sampling inspection is normally performed in which an automobile wheel is cut at the welded portion to measure the actual welding depth at the cut surface. In this method, however, the automobile wheel used in the inspection is discarded to increase the production cost in accordance with the proportion of samples. In addition, this method requires various work to be performed sequentially, such as cutting an automobile wheel, polishing the cut surface, and measuring the actual welding depth, which require a relatively large amount of time and effort.
The present inventors propose a non-destructive inspection method that uses ultrasonic waves to inspect a welded portion as described in Japanese Unexamined Patent Publication No. JP2007-101329. In such a method, ultrasonic waves are transmitted to a welded portion and reflected waves are received to extract second-order harmonic waves, and interface-reflected waves reflected by the interface of the welded portion are found out from the second-order harmonic waves to measure the interface depth of the welded portion. According to this method, the interface depth of the welded portion can be measured accurately to allow non-destructive inspection in order to find the greatest welding depth at the welded portion. Further, the interface depth of the welded portion can be measured at each measurement position by changing the measurement position at which ultrasonic waves are transmitted and reflected waves are received to make it possible to know the interface shape of the welded portion on the basis of each interface depth measurement value.